Exhaust gas emitted from an internal combustion engine such as an automobile engine contains carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), and so forth. These detrimental substances are generally purified by an exhaust gas purification catalyst in which a catalyst component mainly consisting of a precious metal such as platinum (Pt), rhodium (Rh), palladium (Pd), iridium (Ir), etc., is supported by an oxide support such as alumina.
To support the precious metal of the catalyst component on the oxide support, a method is generally used which involves the steps of using a solution of a precious metal compound optionally modified, allowing the oxide support to be impregnated with this solution so as to disperse the precious metal compound on the surface of the oxide support, and baking the oxide support. Materials having a high specific surface area such as gamma-alumina are generally employed for the oxide support to give a large contact area with the catalyst component to the exhaust gas.
It is known that the performance of supported metal catalysts depends on the structure and composition of the metal nanoparticles they contain, and the nature of the support.
Though simple, conventional impregnation methods used for the preparation of supported catalysts often provide limited control over the structure of the resulting materials (i.e. average particle size, particle composition and location of the active components).
In order to overcome such disadvantages, published literature describes the use of alternative synthetic routes such as the use of organometallic molecular carbonyl cluster precursors as well as methods involving the use of templating agents (e.g. surfactants and polymers). The potential advantages of using metal carbonyl clusters as precious metal source for catalytic applications lay in the high metal dispersions and homogeneity in particle size composition due to the relatively low temperature of the activation procedure and, when using heterometallic cluster precursors, to the preformed heterometallic bonds. However, the limited stability of such clusters on the surfaces of various supports, as well as difficulties in their synthesis and handling makes the use of cluster-derived catalysts problematic for large-scale applications.
On the other side, synthetic routes based on the use of templating agents offer the possibility to prepare colloidal metal nanoparticles with controlled particle size and composition. The synthetic steps for the preparation of supported metal catalysts through colloidal routes commonly involve the interaction between the metal precursors and the protective agent followed by a reduction treatment leading to the formation of a metal colloidal suspension. Such metal suspension can be then deposited onto the support surface and finally the protective agent removed to expose the nanoparticles to the reactants.
Few examples are reported in the literature describing the use of polymer-stabilized precious metal colloids as precursors for the preparation of supported metal catalysts where improved metal dispersions with respect to conventional methods are achieved.
Liu et al. (Polym. Adv. Technol. 1996, 7, 634) describe the deposition of Poly vinylpyrrolidone—(PVP) and polyvinylalcohol—(PVA) protected Pt and Pd nanoparticles on SiO2 surface. However, such surface needed to be pretreated by pre-adsorption of poly acrylic acid to ensure deposition of the polymer-capped nanoparticles.
Pd colloidal suspensions were prepared by Burton et al. (Top. Catal. 2008, 49, 227-232) by heating up to 300° C. a suitable Pd precursor in a triclyphosphine or in an octylamine solution. The obtained particle were then washed with hexane and deposited onto an oxidic support followed by calcination of support in order to remove the capping agent.
Higher purification performance of the exhaust gas has been further required for such an exhaust gas purification catalyst for the environmental protection. Control of the cluster size of the precious metal to an optimal size is one way. According to the supporting method of the precious metal of the prior art which uses a solution of the precious metal compound, the precious metal is adsorbed on the oxide support at an atomic level in which the precious metal compound is dispersed to the surface of the oxide support, but the atoms of the precious metal move and invite grain growth in the baking process in which the precious metal is firmly supported. It has therefore been extremely difficult to support only the precious metal of a desired cluster size on the oxide support.
Japanese Unexamined Patent Publication (Kokai) No. 2003-181288 proposes a method for supporting a precious metal on an oxide support by introducing the precious metal into pores of a hollow carbon material such as a carbon nano-horn or a carbon nano-tube so that the precious metal forms a cluster having a desired size, instead of directly supporting the precious metal on the oxide support, fixing the precious metal to the carbon material, then baking them together and thereafter burning and removing the carbon material and at the same time, supporting the precious metal on the oxide support.
According to such a method, the precious metal exists inside the pores of the carbon material until the carbon material is burnt and removed, and when the carbon material is burnt and removed, the precious metal is quickly supported on the oxide support. Therefore, the precious metal can be substantially supported by the oxide support at a cluster size inside the pores of the carbon material. However, this method is not free from problems in which the precious metal must be introduced into the pores of the hollow carbon material, which results in low productivity.
Torigoe, Esumi et al. proposes in “Chemical Industry”, pp. 276-296 (1998) to produce precious metal particles having particle sizes in the order of nm by reducing a mixed solution of a polymer compound such as polyvinyl pyrrolidone and precious metal ions by using a reducing agent such as H2, NaBH4, C2H5OH, or the like.
However, when a compound is used as the reducing agent in the method described above, there is a problem that an element or elements are contained in the compound mix as impurities in the final precious metal particles. When NaBH4 is used as the reducing agent, for example, Na and B mix. When an alcohol is used as the reducing agent, not only the alcohol, but also ketones, aldehydes, carboxylic acids, etc., formed while the alcohol is oxidised during the reduction of the metal ions, may mix. When hydrogen is used as the reducing agent, problems occur in that the particle diameter of the resulting precious metal particles becomes large and the particles are odd-shaped.
WO 2004/089508 provides a method of preparing an oxidation catalyst for oxidizing volatile organic fraction and a catalyzed wall-flow filter for use in removing soot particulates from diesel engine exhaust, including preparing a PGM salt and a transition/alkali metal salt with a water-soluble polymer compound and a reducing agent, to obtain a first colloidal solution, which is then washcoated to a catalyst-support-coated monolithic ceramic substrate, followed by calcination process at high temperatures, to obtain an oxidation catalyst; and treating a PGM salt and a metal salt mixture including at least one selected from a first group of catalyst metal to increase oxidation activity for nitrogen monoxide (NO) and at least one selected from a second group of catalyst metal to decrease a combustion temperature of soot particulates by oxidizing agents, such as nitrogen dioxide and oxygen, with a water-soluble polymer compound and a reducing agent, to obtain a second colloidal solution, which is then washcoated on a catalyst-support-coated wall-flow filter, followed by calcination process at high temperatures, to obtain a catalyzed wall-flow filter.
WO 95/32790 relates generally to the control of hydrocarbons, carbon monoxide, and nitrogen oxides in the exhaust of internal combustion engines. More particularly, the invention relates to the removal of NO when the exhaust gases include oxygen substantially in excess of that needed for combustion of the fuel. This is for example the case with lean burn engines, diesel engines, and other engines currently under development.
US 2008/0268159 relates to a production method of a precious metal catalyst. More specifically, the present invention relates to a production method of a precious metal catalyst the cluster size of which is controlled. US 2008/0628159 provides a production method of a precious metal catalyst including the steps of uniformly mixing a solution containing a precious metal and an aqueous solution of a polymer compound capable of coordination with the precious metal to form a complex of the precious metal and the polymer compound, adding the drop-wise aqueous solution containing the complex to water containing micro-bubbles containing therein hydrogen, mixing the solutions to reduce the precious metal, supporting the mixed solution on a support and baking the solution.
The processes known from the state of the art have several disadvantages, like for example the use of several steps procedures to obtain the final catalyst, limited control over the colloidal nanoparticles location upon impregnation on a support-coated wall-flow filter, the use of high temperature treatment for the formation of the colloidal suspension or the use of a H2 micro-bubble generator, which have limited life time in solution. These disadvantages limit the applicability of the method and the productivity.
The present invention provides a process for preparing a catalyst avoiding the disadvantages of the processes known from the state of the art.